Best E-Mail Converter Tools for Outlook, Gmail & More

E-Mail Converter Guide: Convert PST, MBOX, EML and HTMLConverting email files between formats is a common need when migrating mailboxes, backing up messages, or making messages readable by different clients and systems. This guide explains the main email file formats (PST, MBOX, EML, HTML), why you might convert between them, common conversion workflows, available tools and techniques (automatic and manual), best practices, troubleshooting tips, and considerations for preserving metadata, attachments, and folder structures.

Quick overview of formats

PST (Personal Storage Table): proprietary Microsoft Outlook format that stores email, contacts, calendars, and tasks. Used on Windows and by Outlook desktop.
MBOX (MailBOX): plain-text mailbox format used by many UNIX/Linux mail clients (Thunderbird, Apple Mail in older versions) where messages are concatenated in a single file.
EML: single-message file format (RFC ⁄₂₈₂₂ style) used by Outlook Express, Windows Live Mail, Thunderbird, and many others. Each file represents one email.
HTML: web-friendly representation of message content for viewing in browsers or embedding in web pages; useful for archival and publishing.

Why convert emails?

Migrate between clients (e.g., Outlook ↔ Thunderbird).
Extract messages for legal discovery or compliance.
Archive emails in standard, long-lived formats.
Recover emails from damaged mail stores.
Make emails readable in web or mobile contexts.
Preserve or export specific messages (for sharing or backup).

Key conversion goals and what to preserve

When converting, aim to preserve:

Message text (plain and HTML bodies).
Attachments and file types.
Timestamps (sent/received).
From/To/Cc/Bcc headers.
Subject line and message IDs.
Folder hierarchy and mailbox structure.
Read/unread flags, categories, and labels where possible.
Inline images and embedded resources.

Note: Some formats store richer metadata (e.g., Outlook PST includes calendars, contacts) that may be lost or need special handling when converting to basic mailbox formats.

Common conversion workflows

Below are practical workflows for frequent scenarios.

1) PST → MBOX or EML (Outlook to Thunderbird/others)

Options:

Use a dedicated converter tool (commercial or open-source) that reads PST and exports MBOX/EML while preserving folder structure, attachments, and timestamps.
Use Outlook + an export plugin/add-on to save messages as EML, then concatenate EMLs into MBOX if needed.
Convert PST to MBOX via an intermediate format like EML or using a script that leverages libpst (readpst) on Linux.

Steps (general):

Back up the PST file.
If using readpst: run readpst to produce mbox files or EML directory.
Import resulting MBOX into Thunderbird (File → Import → Mail).
Verify folder tree, message counts, attachments, and dates.

2) MBOX → PST (Thunderbird to Outlook)

Options:

Use import tools (commercial converters) that create PST from MBOX directly.
Use Thunderbird with an add-on (like ImportExportTools) to export messages as EML, then drag them into Outlook (which can import EML), or use an MBOX-to-EML-to-PST pipeline.
Use third-party utilities to convert MBOX to PST, ensuring Unicode support for non-Latin characters.

Steps (general):

Export MBOX from source client.
Convert with a trusted tool to generate PST.
Open PST in Outlook and verify.

3) EML ↔ HTML (single-message exports and web views)

EML to HTML: Parse the EML to extract headers and HTML body, save inline images and attachments, and rewrite resource links.
HTML to EML: Wrap the HTML body in proper RFC 822 headers and encode attachments in MIME parts.

Tools: many mail clients can “Save as” HTML or EML; command-line tools and libraries (Python email, Node mailparser) can automate bulk conversions.

Tools and utilities

readpst / libpst (Linux): extract mailboxes from PST to mbox/EML. Good for many PST files; watch out for encoding issues.
Thunderbird + ImportExportTools NG: flexible for MBOX/EML tasks.
Outlook (desktop): native PST support; can export in various ways, or use add-ins.
Commercial converters: numerous options that promise better metadata preservation, GUI, and batch conversions (evaluate by testing with sample data).
Programming libraries: Python (email, pypff), libpff for PST, mailparser, Node.js mailparser — useful for custom conversions and automation.
Online converters: convenient for small/one-off tasks; avoid sending sensitive mail to online services without ensuring privacy/compliance.

Preserving metadata and folder structure

Test converters on a small mailbox copy first.
Ensure destination format supports the metadata you need; otherwise plan for metadata export (CSV/JSON) alongside messages.
For folder hierarchy, export with tools/options that keep folder paths, or export into a directory tree where each folder becomes a subdirectory of EML files or an MBOX per folder.

Handling attachments and embedded content

Save attachments as separate files alongside exported messages when possible.
For HTML exports, rewrite inline content to reference local copies and keep a manifest of attachments.
Beware of large attachments — use batch conversion with throttling to avoid memory/timeouts.

Encoding, character sets, and internationalization

UTF-8 is preferred for cross-platform compatibility.
When converting from legacy encodings (ISO-8859-*, Windows-125x), ensure the tool correctly interprets and transcodes characters.
Verify subject lines, headers, and body text in several messages after conversion.

Automation and scripting tips

For bulk jobs, script conversions using command-line tools or libraries, and log results.
Validate counts: compare message counts and total size before and after conversion.
Implement retries for transient I/O failures and isolate problematic messages for manual review.

Example (readpst command):

readpst -r -o output_dir mailbox.pst

This extracts PST contents recursively into an output directory as mbox/EML files.

Troubleshooting common problems

Missing messages: check for corrupted PST or size limits; run integrity checks and try alternate tools.
Broken attachments: ensure MIME parts are preserved; check for transfer-mode issues (binary vs. text).
Incorrect dates: watch for timezone or epoch-handling bugs; some tools add offsets.
Characters displayed as � or garbled: indicates encoding mismatch; re-run conversion with explicit charset handling.
Folder structure flattened: use a converter that preserves hierarchy or export per-folder MBOX/EML.

Security and privacy considerations

Treat exported mail as sensitive data. Keep conversions on a secure, offline machine when possible.
For legal/forensic work, maintain chain-of-custody and use verified tools.
Avoid online converters for sensitive mail unless the service meets your compliance and privacy needs.

Choosing a converter: quick checklist

Does it preserve the metadata you need (timestamps, flags, headers)?
Does it support attachments and embedded content correctly?
Can it handle large mail stores and batch processing?
Does it keep folder hierarchy?
Is it compatible with your character sets and encodings?
Do you need GUI simplicity or command-line automation?
What’s the vendor reputation or open-source project activity?

Requirement	PST → MBOX/EML	MBOX → PST	EML ↔ HTML
Folder hierarchy	Good with dedicated tools	Good with dedicated tools	N/A / per-message
Attachments preserved	Usually yes	Usually yes	Requires mapping
Metadata fidelity	Varies by tool	Varies by tool	Limited
Automation-friendly	Yes (CLI/libs)	Yes	Yes

Example migration scenario

You have an Outlook PST from an ex-employee and need to import it into Thunderbird while preserving folders and attachments.

Copy the PST file to a secure workstation.
Install readpst (or a trusted commercial PST extractor).
Run readpst to produce per-folder MBOX files: readpst -r -o ./output mailbox.pst
Open Thunderbird and import each MBOX (or place them into Thunderbird’s profile Mail folder and restart).
Spot-check message counts, attachment presence, and headers.
If issues appear (encoding or missing fields), try a different converter or export as EML and re-import.

Final notes

Always work from backups; never run destructive operations on original mail stores.
Test multiple tools on sample data before committing to a full migration.
Keep a log of conversions, counts, and known issues for auditing and rollback.